Typically, a photodiode used for an optical element is an optical sensor receiving an electrical signal (i.e., current or voltage) from an optical signal by converting optical energy into electrical energy. A photodiode can be a semiconductor element in which a junction unit of the photodiode has an optical detection function. Such a photodiode utilizes the principle that excessive electrons or holes are generated by photon absorption, thereby modulating the conductivity of the photodiode. That is, the electric current produced by a photodiode varies according to the generation of carriers in response to incident photons. Such a characteristic of a photodiode provides a method of converting optical signals into electrical signals. By converting optical signals into electrical signals, a photodiode can be used as a light-receiving element of a photo pickup device. The photo pickup device reads data or information from optical recording devices such as a CD, a DVD, a DVD R/W, COMBO, COMBI, or Blue ray. A photodiode integrated chip (PDIC) formed by combining a photodiode with a circuit unit is under development. In the PDIC, a signal may be processed by a preamplifier.
Examples of photodiodes include a P-N junction photodiode, a PIN (P type electrode-intrinsic epitaxial layer-N+ type layer-P substrate) photodiode, an NIP (N type electrode-intrinsic epitaxial layer-P+ type layer-P substrate) photodiode, and an avalanche photodiode (APD) using an avalanche multiplication effect. A P-N junction photodiode has a low response speed and bad frequency characteristics. An APD generates noise and consumes a large amount of power. Therefore, PIN photodiodes and NIP photodiodes are typically used.
The performance of a photodiode is evaluated by photoefficiency and frequency characteristics (i.e., bandwidth). A photodiode can achieve high performance if the photodiode has high photoelectric efficiency for wavelengths of detected light and a sufficient response speed.
Conventionally, an intrinsic epitaxial layer of a PIN photodiode or an NIP photodiode is formed to have a particular resistivity and a thickness using a single thin film growth method. When the doping density of the intrinsic epitaxial layer (which may not be an intrinsic semiconductor) increases, the number of current carriers increases, thus improving frequency characteristics. When the doping density of the intrinsic epitaxial layer increases, the probability of electron-hole plasma (EHP) recombination also increases, thus decreasing the photoefficiency of a photodiode. Thus, there is a trade-off relationship between photoefficiency and frequency characteristics of a photodiode. The performance of a photodiode may be improved by optimizing the thickness of an intrinsic epitaxial layer and the density of impurities.
When an intrinsic epitaxial layer is formed using a single thin film growth method, impurities may be out-diffused from a high density P type layer in upper or lower regions of an intrinsic epitaxial layer into an intrinsic epitaxial layer. The impurities may be diffused from a high density N type layer in upper or lower regions of an intrinsic epitaxial layer into an intrinsic epitaxial layer. That is, a high-temperature heat treatment (during an epitaxial layer fabrication, subsequent annealing, or baking) may be performed to form a photodiode or an element including a photodiode. During such a heat treatment, impurities such as a dopant diffuse from the high density P type or N type layer to the intrinsic epitaxial layer. The diffusion of impurities into the intrinsic epitaxial layer decreases the performance of a photodiode.
Referring to FIG. 1, a sectional view of a conventional NIP photodiode is shown. A high density P-type buried layer 2, a P type first intrinsic epitaxial layer 3, an N type second intrinsic epitaxial layer 5, and an N+ type high density layer 8 for contacting with a cathode are formed on a P type semiconductor substrate 1. A first P type junction region 4 is formed in the P type first intrinsic epitaxial layer 3. A second P type junction region 6 is formed in the N type second intrinsic epitaxial layer 5. A P+ type layer 9b contacting with an anode formed in the P type junction region 6 contacts a metal distributing structure 11 of an anode electrode. A P+ type dividing layer 9a is formed on the N type intrinsic epitaxial layer 5 to divide a light receiving unit of a photodiode. The metal distributing structure 11, which is an anode electrode, is insulated by an interlayer insulating layer 10 and an intermetal insulating layer 12. An element separating layer 7 formed by Local Oxidation of Silicon (LOCOS) electronically separates a photodiode from adjacent elements. A silicon oxide (SiO2) layer 14 and a silicon nitride (SiN) layer 15 form an anti-reflective coating (ARC) 16 for blocking the reflection of light projected on the light-receiving unit of the photodiode.
Growing the epitaxial layers 3 and 5 can be performed in a high temperature of about 1100□ to about 1150□ to fabricate the conventional NIP type photodiode shown in FIG. 1. Forming a subsequent element can be performed in a heat treatment such as annealing or baking. The heat treatment out-diffuses impurities from the high density P-type buried layer 2 into the P type first intrinsic epitaxial layer 3. The heat treatment diffuses impurities from the high density N+ type layer 8 into the N type second intrinsic epitaxial layer 5. Such diffusion or out-diffusion decreases the performance of the photodiode. That is, when impurities are diffused to the intrinsic epitaxial layers 3 and 5 due to a high-temperature heat treatment, the thicknesses of the intrinsic epitaxial layers 3 and 5 are substantially reduced, and the capacitances of the intrinsic epitaxial layers 3 and 5 are increased. As a result, frequency may be reduced. The diffusion of the impurities decreases a depletion layer, thus also decreasing photoefficiency. A conventional PIN photodiode has similar problems with a conventional NIP photodiode.